Op, what you've described is exactly the reason I stayed in a bad marriage - I knew he'd fight for 50:50 and j wouldn't be there to protect them. I had to step in so many times and he even actually marked DC when hitting too hard.
But nobody did anything even though marking a Dc is illegal. Social workers were concerned about his parenting and put Dc as child in need status for a while. Police took no action as there was no proof and no witnesses, and they said a judge would turn a blind eye to just one or two episodes, especially if he claimed ignorance and regret (which he did).
But then he didn't like that I began to stand up to him and he got rid of me in terms of ending the relationship. And he went after 50:50.
I discussed with a domestic abuse organisation who organised a formal restraint order forbidding certain behaviours towards me and dc, and occupation order but both were very hard to get and the latter took a long time - we lived together while it was going ahead can you believe it, which was difficult but more tolerable than being apart because i could still be there for DC and it was better than leaving them alone together.
Eventually it went to court and DC now reside with me. The judge was critical and i think because the father doesn't see Dc as much as he wanted to, he makes an effort during their short time together. It's a shame because it's so acrimonious and i would prefer it not to be, but it's not my choice it's his.
My DC's age was a lot older than your two though, and that itself is a comfort to know they'd be likely to protest now. And vote with their feet by refusing contact. I don't know how that will go for younger children other than to say it WILL impact on them and it might be that it has to get worse before they can be properly protected. They went through a lot when we all lived together. Saw and heard things no child should see, hear or endure. I would have much rather protected any child of mine from this damage but I couldn't. You might not be able to either. But do try. Give it your all. Nobody else will if you don't and it might pay off, especially if they eventually complain themselves.
But it's a minefield as the new fathers right amendment trump card is parental alienation and they don't even need proof to accuse you of it. Judges seem to err on the side of more leniency to Dad than Mum on this one, so make sure you don't even appear to be negative about Dad. Don't gaslight your kids either though. Keep it real but restrained. And such good luck to you. I'm so sorry you're walking a path I’ve been down. They are dark days but I promise you it does pass and the dawn breaks eventually.
I now live in a better home, in an areas I prefer, closer to school and my family and friends and better then that, my DC sleeps under my roof safe. And DC can visit Dad and get the best of him, as, like I say, eventually he missed DC enough to make an effort now. Oh, and, importantly, there is something magical about knowing they're safe now - a child can begin to heal and they are so resilient that way, it happens fast. We did need counselling help though, which you can get through GP or school.